Reflow soldering is a form of soldering that enables reflowable (e.g., surface mounted) components to be efficiently and effectively mounted to one or both side(s) of a circuit card. The term "circuit card" generally describes any substrate, with or without a component, for accommodating a circuit. A circuit card would include but is not limited to a printed circuit board ("PCB"), a printed circuit assembly ("PCA"), and a bread board. FIGS. 1A to 1C depict a circuit card assembly 10 having a circuit card 11, an edge connector 14, and reflowable components 17.
The reflowable components 17 are soldered to component pads 12 (FIG. 1C) of circuit card 11. Typically, solder paste is screenprinted onto the component pads 12; components 17 are then placed onto their corresponding pads atop the solder paste. After this placement, the entire card (including components 17) are heated in an oven to reflow the solder, i.e., to melt and separate the solder from the solder paste so as to create both a mechanical and electrical connection between components and their component pads after the reflowed solder has cooled and solidified.
Reflow soldering is well-suited for the at least partially automated manufacture of printed circuit assemblies. Reflowable components, which can be placed essentially vertically onto screenprinted solder paste, pierce the solder paste deposit without displacing it from the pad on which it is deposited. When reflow heating occurs, both the component leads and the solder paste are properly positioned. Unfortunately, it has not been practical to mount certain other nonvertically approaching components such as edge connectors onto the circuit card with reflow soldering methods.
With reference to FIGS. 1A and 1B, edge connector 14 has a connector body 15 and nonvertically approaching leads 16. FIG. 1A shows connector 14 prior to being mated with the circuit card 11. Conversely, FIG. 1B shows connector 14 after it has been mated with the circuit card 11.
Edge connector 14 is a "nonvertically approaching device" because it has nonvertically approaching leads 16. A "nonvertically approaching lead" is a lead that when being positioned onto a pad for soldering is not amenable to being placed onto the pad in a vertical fashion. For example, an edge connector 14 (as depicted in FIGS. 1A and 1B) normally has two closely spaced rows of connector leads 16 that are designed to straddle an edge of the circuit card 11 and conductively contact connector pads 13, which are located at the edge of the circuit card. Based on the geometrical configuration of the connector with respect to its nonvertically approaching leads 16, it is typically not viable to mate the connector 14 with the circuit card 11 so that the leads 16 encounter the solder paste with a vertical approach; rather, the leads 16 approach horizontally, somewhat parallel to the plane of the connector pads 13 and move across the connector pads at or very close to the connector pad surfaces.
Thus, as is depicted in FIGS. 2A and 2B, when a nonvertically approaching device such as edge connector 14 is mated to a circuit card after solder paste 21 has been applied to its pads in preparation for reflow, its nonvertically approaching leads 16 tend to "scrape" or displace the solder paste off of the pads before reflow. When reflow occurs, the solder joints are often unsatisfactory due possibly to shorting between pads, insufficient solder at the lead and pad interface, or solder balling.
In addition, as shown in FIGS. 2C to 2E, a further problem may arise. In some automated processes (e.g., with a circuit card having components on both sides), the solder paste deposited on a primary side is reflowed before solder paste is spread onto the secondary side. This means that solder paste spread on primary side connector pads will reflow, cool, and solidify atop the primary side connector pads before solder paste is spread onto the secondary side connector pads and thus, before the edge connector 14 is mated with the circuit card 11. Now, when the edge connector is mated to the circuit card prior to reflow heating, not only is solder paste 21 scraped off of the secondary connector pads by the horizontal approach of the leads 16, but also, the primary side solder domes 23 cause the primary side leads of the edge connector 14 to deflect away from their target connector pads. When reflow occurs, the resulting solder bonds on both the primary and secondary sides are often unsatisfactory.
To avoid such problems, nonvertically approaching devices have been manually soldered to circuit cards after the reflowable components have been mounted. During the solder paste deposit phase, solder paste is not applied to the pads corresponding to the nonvertically approaching devices. Once the reflowable components have been reflowed, a human operator manually mates the nonvertically approaching device with the circuit card. Solder is then "dragged" across each nonvertically approaching lead with a soldering iron that may have a special finepoint tip. It can be seen, however, that this method is tedious and time consuming for the human operator. Moreover, such manual soldering can lead to poor solder connections resulting from human error, as well as from inherent flaws in the soldering method. For example, cold solder connections can occur due to heat being drawn away from the soldering surface by the surrounding structure (e.g., connector, circuit card, work surface), which are at room temperature. In addition, the dragging of solder across a lead may merely "blanket" the lead without sufficiently "wetting" (i.e., bonding of the solder to the component lead and component pad surfaces).
Accordingly, what is needed in the art is a solution for operably mounting a nonvertically approaching device such as an edge connector onto a soldering surface with a reflow soldering method.